It is known to manufacture hollow metallic aerofoils for example to be used as blades in a jet engine, and in particular fan blades for a turbomachine, by superplastic forming and diffusion bonding metallic panels, the panels forming pressure and suction surfaces of the blade. These blades are generally referred to as wide-chord fan blades. The metallic panels may include elementary metal, metal alloys and metal matrix composites. At least one of the metallic panels must be capable of superplastic extensions. In one known process the surfaces of the panels to be joined are cleaned, and at least one surface of one or more of the panels is coated in preselected areas with a stop-off material to prevent diffusion bonding. The panels are arranged in a stack and the edges of the panels are welded together, except where a pipe is welded to the panels, to form an assembly. The pipe enables a vacuum, or inert gas pressure, to be applied to the interior of the assembly. The assembly is placed in an autoclave and heated so as to “bake out” the binder from the material to prevent diffusion bonding. The assembly is evacuated, using the pipe, and the pipe is sealed. The sealed assembly is placed in a pressure vessel and is heated and pressed to diffusion bond the panels together to form an integral structure. Diffusion bonding occurs when two mating surfaces are pressed together under temperature, time and pressure conditions that allow atomic interchange across the interface. The first pipe is removed and a second pipe is fitted to the diffusion bonded assembly at the position where the first pipe was located. The integral structure is located between appropriately shaped dies and is placed within a rig. The integral structure and dies are heated and pressurised fluid is supplied through the second pipe into the interior of the integral structure to cause at least one of the panels to be superplastically formed to produce an article matching the shape of the dies.
In addition to the hollow structure just described, it is also known to insert a membrane 2 between the metallic panels 4, 6 prior to the above described process (see FIG. 1 for example). The location of diffusion bonds between the membrane and the adjacent panels can be controlled by applying the stop-off material to preselected areas on each side of the membrane (or respective panels). When the aerofoil is subsequently expanded, at points where the membrane is attached to the outer panels this produces the internal structure.
FIG. 1 illustrates a known method of manufacturing an aerofoil structure. In this method a metallic sheet 2, made from, for example, titanium, is provided for forming the aerofoil structure. The sheet 2 is forged so as to produce two sections 4 of the sheet which are thicker than the main body of the sheet 2. This may be achieved by upset forging, wherein the length of the sheet is reduced in order to obtain the desired increase in cross-section or by drawing the sheet so as to increase the length and reduce the thickness of the main body of the sheet. The sheet 2 may be machine finished prior to the forging and/or splitting/dividing process.
As illustrated in FIG. 1(c) the forged sheet 2 is divided along an inclined plane 6 extending in a span-wise direction, so as to produce two substantially identical panels 8 and 10. The panels 8 and 10 need not be identical and alternative configurations may be used in order to provide desired properties for the resulting aerofoil e.g. the panels may be of differing thickness. The sheet 2 may be divided into the two panels 8 and 10 by way of any known technique which may be suitable for cutting the required width of the aerofoil, for example by using a band saw. To facilitate the cutting process, channels may be machined inboard of the elements 4 to allow the band saw to enter the workpiece, as described in patent application GB2306353 (see FIG. 2). The resulting panels 8 and 10 taper from the section 4 to the tip end of the panel.
The two panels 8 and 10 are then assembled so that their uncut exterior surfaces (which have been machine finished) are facing each other. Optionally, a membrane 12 may be positioned between the panels 8 and 10. As previously described, the assembly may then be diffusion bonded and superplastically formed in order to produce the desired external shape of the aerofoil. When joined, the sections 4 combine to form the root of the aerofoil which serves in use to attach the aerofoil, for example, to the hub of the rotor.
This method of manufacturing an aerofoil structure has certain disadvantages attributable to the forging process which is necessary in order to obtain the sections 4 that form the root of the aerofoil once the two panels 8 and 10 are assembled. In particular the thickness of the root is limited by the forging process such that it is often not possible to produce a root with the required thickness for the application. Furthermore, since the root and aerofoil structures experience different working loads and environments, it is desirable for these structures to have different material properties. This is not possible in the prior art method.
In accordance with a first aspect of the present invention there is provided a method of manufacturing an aerofoil structure capable of being diffusion bonded and superplastically formed to create a substantially hollow cavity within the aerofoil structure, the method comprising: providing two metallic panels; assembling and joining the two metallic panels to one another to form the aerofoil structure; wherein the two metallic panels each comprise a surface capable of forming an aerofoil and further comprise a root or section thereof which is either integral with or fixed to the aerofoil surface; and incorporating a section of a different material into a part of the aerofoil structure the said section being made from a material which is different from the material of another part of the aerofoil structure.
The section of a different material may be provided as a layer on the metallic panels. The method of manufacturing an aerofoil structure may further comprise selecting the different material to provide desirable properties for the section.
The section of a different material may be incorporated into one of the metallic panels or aerofoil surfaces, one of the metallic panels or aerofoil surfaces being manufactured from a lower quality material and the other one of the metallic panels or aerofoil surfaces being manufactured from a higher quality material.
A lower quality metallic panel or aerofoil surface may be manufactured from two pieces of the lower quality material and the higher quality metallic panel or aerofoil surface may be manufactured from a single piece of higher quality material.
The section of a different material may be incorporated into the leading and/or trailing edge of the aerofoil surface in order to increase service life.
The section of a different material may be incorporated into the tip of the aerofoil in order to increase strength of the tip to withstand contact with a casing abradable.
One of the aerofoil surfaces may be a pressure surface and the other aerofoil surface may be a suction surface, the suction surface being manufactured from a recycled material.
The section of a different material may be incorporated into a flank of the root in order to impart resistance to fretting of the root.
The section of a different material may be incorporated into the root so that the strength of the root material is greater than that of the aerofoil surfaces.
The root may be joined to the aerofoil at a position below an annulus line.
In accordance with a second aspect of the present invention there is provided a substantially hollow aerofoil structure comprising two metallic panels, each metallic panel comprising a surface capable of forming an aerofoil and further comprising a root or section thereof which is either integral with or fixed to the aerofoil surface; the aerofoil structure further comprising a section of a different material incorporated into a part of the aerofoil structure, the said section being made from a material which is different from the material of another part of the aerofoil structure.
The section of a different material may be provided as a layer.
The section of a different material may be incorporated into one of the metallic panels or aerofoil surfaces, one of the metallic panels or aerofoil surfaces being manufactured from a lower quality material and the other one of the metallic panels or aerofoil surfaces being manufactured from a higher quality material.
The lower quality metallic panel or aerofoil surface may be manufactured from two pieces of the lower quality material and the higher quality metallic panel or aerofoil surface may be manufactured from a single piece of higher quality material.
The section of a different material may be incorporated into the leading and/or trailing edge of the aerofoil surface.
The section of a different material may be incorporated into the tip of the aerofoil.
One of the aerofoil surfaces may be a pressure surface and the other aerofoil surface is a suction surface, the suction surface being manufactured from a recycled material.
The section of a different material may be incorporated into a flank of the root.
The section of a different material may be incorporated into the root.
The root may be joined to the aerofoil at a position below an annulus line.